E. coli outbreak blamed on lettuce leaves

The UK’s Food Standards Agency has said that lettuce leaves are behind an outbreak of E. coli that has left hundreds of people unwell. It has forced some of the UK’s biggest supermarkets to withdraw some sandwiches containing salad leaves. To find out more about the bacteria behind this intestinal infection, I went to meet Nicholas Brown who is a consultant microbiologist at Addenbrooke’s Hospital, Cambridge…

Nick - Over the last few weeks, there have been a number of cases of haemolytic uremic syndrome and also children with bloody diarrhoea presenting in different parts of the country. And this has been picked up through national surveillance, both of cases, clinical cases, diagnosed in hospitals, but also through laboratory surveillance of isolates of shiga toxin producing E. coli. These have been linked together because when they have been analysed genetically, they have been indistinguishable, which suggests that there has been a common source. Though the fact that they are so geographically dispersed makes it difficult to know what that single source is and therefore makes it more difficult to control it.

Chris - It's a form of E. coli, but where will it have come from? Because all of us carry e coli. So what's different about this one and why is it doing what it's doing?

Nick - Absolutely. So E. coli is one of the commonest bugs that we have associated with particularly our gut. It is found in faeces of all humans and animals, and most of us carry it completely innocuously without any problem at all. However, some strains of E. coli carry virulence factors, which can cause more severe disease and a variety of different diseases too. So some strains are associated with urinary tract infections, some strains associated with meningitis in neonates, and others causing gastrointestinal problems. So that shiga toxin producing strains are just one subgroup of E. coli that are particularly virulent and can cause bloody or hemorrhagic colitis and sometimes severe complications such as haemolytic uremic syndrome, particularly in children.

Chris - Where will they come from? What normally carries them?

Nick - They are carried within the gut of mainly animals and therefore most cases are associated with contact with animals or sometimes petting farms situations where we might come into contact with animal faeces and then through the hand-mouth route, faecal-oral spread, then get it into our guts where it can cause disease.

Chris - How's it got into the food chain then? Because the finger is being pointed at some kind of food source for this.

Nick - It would seem likely therefore that there would be some sort of faecal contamination of the raw material, the food at source. And clearly a lot of food stuffs come from farms where there is also animal livestock as well. And so therefore it would likely be a breakdown of basic hygiene at some point in the food manufacturing process, although difficult to say exactly where.

Chris - So it need not be just the meat of an animal. It could be if some salad vegetables have animal faeces on them and those salad leaves or whatever have gone into a sandwich, you could get it from either of those two routes.

Nick - Absolutely, yes. And, that is why it's so important that we wash raw foods that we eat without any further processing, such as salads.

Chris - And how do we manage the cases that we detect?

Nick - Most of the treatment of these cases is what's called supportive. There is no particular antibiotic therapy required or indeed appropriate. And the patient's symptoms are managed as they occur with appropriate fluid resuscitation and most cases of diarrhoea resolve by themselves. With time.

Chris - Anybody particularly vulnerable?

Nick - Young children are particularly more likely to develop the severe complications of shiga toxin producing E. coli, which is hemolytic uremic syndrome, which is a kidney disease sometimes requiring support with dialysis and on occasion support in a paediatric intensive care unit. That's very much the tip of the iceberg with the most severe cases. Majority of people with this infection will not develop that and will simply present with diarrhoeal symptoms.

Chris - So how will public health teams now act on this and how have they been acting on it and what will eventually happen?

Nick - The mainstay of the initial investigation is surveillance, putting together the pieces of the jigsaw puzzle to try and identify what the source is. Most of the outbreaks that we see in this country are associated with contact with farms. However, my understanding is that the current outbreak is not, and the fact that the cases are so geographically dispersed would obviously point against that. So we are looking at a likely common source with something that is distributed across the UK and hence the focus on food. What would happen next with such an investigation is that cases would have a detailed food history taken from them. Clearly this is not a hundred percent reliable because people's memory of what they've eaten two weeks ago is often not accurate. But by piecing together the details that we get from those food histories, sometimes it's possible to develop a hypothesis about what sort of food might be responsible.